in use, in lead-acid batteries, the lead grids of the positive plates are subject to far more corrosion and growth than are the grids of the negative plates. In use, the lead in the positive plates corrodes or erodes away and disappears which decreases and eventually ends the useful life or life cycle of the battery. In use, the lead grids of the positive plates also grow and expand or elongate, particularly in length and height, on the order of 5% to 15% of their original length and height. This tends to cause the positive grid plates to pierce their separators or envelopes and short or ground out with an adjacent negative plate. This growth also tends to limit the design or pattern of the wires of the positive grids because diagonal wires are longer and hence have a greater absolute increase in length and hence a greater tendency to pierce a separator and short out.
Typically, lead battery grids may be made by (1) continuous casting of a web having a plurality of interconnected grids which are subsequently separated, (2) book molding or casting an individual grid or a panel of two grids in a mold usually of cast iron or steel which is water cooled or (3) by an expanded metal process. It has been found that compared to book molded and expanded metal grids, when continuous cast grids are utilized in positive plates, they are subject to greater growth and significantly increased corrosion which results in a useful life in service of about one-half to two-thirds of book molded and expanded metal grids as determined by SAE J-240 life cycle tests. Consequently, for many lead-acid battery applications, such as automotive batteries, continuously cast grids are unsatisfactory for positive battery plates or electrodes. Nevertheless, primarily due to reduced manufacturing and assembly costs, continuously cast lead grids are utilized in the negative plates or electrodes of many automotive batteries.
Therefore, various processes and equipment have been developed and commercially utilized for making continuously cast negative grids and battery plates. Typically, a plurality of grids are cast as a continuous strip or web of lead which may be washed to remove molding compounds and lubricants and is usually coiled for inspection and storage before further processing. Subsequently, the web is uncoiled, passed through a continuous pasting machine, cut into individual pasted grids or plates which are passed through a drying oven to remove excess moisture from the paste, and then stacked in piles of individual plates by a stacker for subsequent processing and assembly as negative plates in lead acid batteries. Suitable continuous casting machines are disclosed in U.S. Pat. Nos. 4,509,381; 4,415,016; 4,534,404; and 4,544,014. A suitable paster is disclosed in U.S. Pat. No. 4,606,383, suitable cutting devices are disclosed in U.S. Pat. Nos. 4,543,863 and 4,583,437, and a suitable stacker is disclosed in U.S. Pat. No. 4,973,218.